What’s that ’n part there?
I personally don't know why they have the "'n" here. Nhw by itself is "They". "Nhw'n" should be a part of "Maen nhw'n" and the "'n" replaces the "yn" because Nhw ends with a vowel.
'Nhw' is 'they', Nhw'n is part from Maen nhw'n and would be better translated as 'they are'.
Maen nhw'n hwyr. -> they are late.
Maen nhw'n barod. -> they are ready.